Many energetic materials, such as pyrotechnic flare compositions, gun propellants, and pressable explosives are manufactured entirely in batch processes. In these batch processes, materials are dried, as required, and weighed into discrete batch size portions for mixing in standard muller-type, vertical planetary mixers, or in horizontal sigma-blade mixers. The ingredients are mixed and further processed into end items. The batch mixing is time consuming, costly, marginally homogeneous, and exposes both operators and equipment to significant hazards from accidental ignition.
For instance, a current process for manufacturing infrared decoy flare compositions uses a muller-type mixer to compound the ingredients. In this process, polyacrylate rubber binder is dissolved in acetone and weighed into a mixer. Fine (20 micron) magnesium powder is added to the mixer and premixed to wet the metal powder. Polytetrafluoroethylene (PTFE) is added to the mix and the slurry is mixed until the acetone evaporates to form a putty-like consistency. When the putty is ready to dump, the operator enters the bay in an aluminized suit (for safety) and places a container under the dump door on the mixer. The mix is remotely dumped to fill the container and the operator removes that container and repeats the process until the mixer is empty. The putty-like composition is spread on trays and placed in large walk-in ovens for complete drying. After drying, the trays are removed and the cakes are broken into chunks that can be granulated for feedstock to the process. The granulating process uses a Stokes granulator which rubs the chunks against a screen. This granulating process has been known to accidentally ignite the pyrotechnic flare composition with subsequent destruction of the facility.
In this process, there are numerous exposures of personnel to bulk quantities of flare composition. This is a significant safety hazard. The drying of bulk quantities of flare composition in ovens is a hazard to facilities and personnel. The mixing time is long since the mixers are not heated and the evaporation of the acetone cools the bowls and retards drying. The Muller-type mixers are not efficient in combining the solids nor in separating the entwined fibers of the PTFE. After granulation, the material is not free-flowing and must be manually weighed, dispensed, and leveled in the dies for consolidation. From start to finish, the process is time consuming and labor intensive requiring several days to complete.
Another process for manufacturing infrared decoy flare compositions uses a "shock precipitation" or "Cowles Dissolver" method. In this process, the binder is dissolved in acetone and placed in a Cowles Dissolver. The PTFE is added and the high-shear mixing of the Cowles Dissolver disperses the PTFE fairly efficiently. The magnesium powder is added to the mixer while mixing, and hexane is added to cause the binder to precipitate from the acetone to coat the solids suspended in the mixture. The mixing is stopped and the solids settle to the bottom. The mixed solvents are decanted, and the solids are washed with additional hexane. The solids are trayed for drying in a large walk-in oven. Again, the batch mixing is labor intensive and results in considerable exposure of personnel and facilities to large quantities of hazardous composition. The process is also time consuming and results in major waste disposal problems.
It will be appreciated that infrared decoy flare compositions are hazardous pyrotechnic formulations that produce extreme heat when ignited. Current batch processes require considerable exposure of personnel and equipment to large quantities of bulk material because it is not possible to operate the process remotely.
Typical composite low vulnerability ammunition ("LOVA") gun propellants, of the type described in U.S. Pat. No. 4,570,540, are prepared in a batch process using a solvent, which requires relatively long processing times and a large number of steps. In a common LOVA gun propellant batch manufacturing process, RDX is dried, ground to a desired particle size, and weighed into a batch size increment for mixing. The other gun propellant ingredients (binder, plasticizer, liquid coupling agent, and stabilizer) are added to a horizontal, sigma blade mixer that has been modified to eliminate seals around the blade shafts. Vertical mixers are precluded from this process because the very high viscosity results in inadequate mixing capability. The ingredients are wet with a mixed ethyl acetate/ethyl alcohol solvent. The materials are mixed for several hours to assure that the organic binder materials are dissolved and coated onto the RDX. The temperature of the mixer is controlled during this entire cycle so that the solvent mixture is not removed prematurely. When the mix cycle reaches a proper time, determined by the amount of mix energy introduced into the propellant, a vacuum is applied and the solvent level is reduced over a period of time to the proper operating level.
The mix is then dumped and transferred to the blocking and straining area. Approximately 60 pounds of LOVA is put into a die and pressed into a cylinder approximately 12 inches in diameter and 16 inches long. The block is placed in a ram extruder and pressed through a sieve plate to put additional work into the propellant to improve mixing. The spaghetti-like strands are collected and re-pressed in the die. The cylinder is transferred to a large ram press with 30 dies. Each die is approximately 0.33 inch in diameter with a 19 perf pin plate to make a perforated grain for the gun propellant. The 60 pound block is extruded in a vertical plane with each strand being collected in a spiral around a cone beneath the die. As the strands exit the dies, the weight of the strands causes an elongation of the strands and a necking down of the diameter. This produces a variable diameter strand that affects the reproducibility of the grains. The solvent content is approximately 10% during extrusion.
The flexible strands are then fed to a rotating blade cutter and cut into pellets approximately 0.5 inches long. The pellets are collected, dried, glazed with graphite to prevent static charges and improve packing, and stored for several weeks to "age" the propellant before it is ballistically accepted. This batch process is costly and very labor intensive. Moreover, the efficiency of the batch mixer produces less than ideal homogeneity and performance reproducibility.
Certain high explosives, such as PAX-type (Picatinny Arsenal Explosive) explosives are processed the same as LOVA gun propellant, except the die is not perforated and the diameter is about 0.09 inches. As in the batch processing techniques described above, homogeneity is a problem. The bulk density of the explosive is controlled by extrusion and chopping of the extrudate, which significantly increases the cost.
From the foregoing, it would be an advancement in the art to provide continuous processing techniques capable of producing high quality, low cost energetic materials. It would be a significant advancement in the art to provide continuous, remotely operated techniques for processing energetic materials and to provide techniques for processing energetic materials which reduce the exposure of personnel and equipment to large quantities of bulk material. It would be yet another advancement in the art to provide energetic materials processing techniques which produce free-flowing granules having a consistent density so that volumetric materials processing equipment may be used in preparing the final energetic composition.
Such energetic materials processing techniques are disclosed and claimed herein.